SUMMARY

Flies rely heavily on visual feedback for several aspects of flight
control. As a fly approaches an object, the image projected across its retina
expands, providing the fly with visual feedback that can be used either to
trigger a collision-avoidance maneuver or a landing response. To determine how
a fly makes the decision to land on or avoid a looming object, we measured the
behaviors generated in response to an expanding image during tethered flight
in a visual closed-loop flight arena. During these experiments, each fly
varied its wing-stroke kinematics to actively control the azimuth position of
a 15°×15° square within its visual field. Periodically, the
square symmetrically expanded in both the horizontal and vertical directions.
We measured changes in the fly's wing-stroke amplitude and frequency in
response to the expanding square while optically tracking the position of its
legs to monitor stereotyped landing responses. Although this stimulus could
elicit both the landing responses and collision-avoidance reactions, separate
pathways appear to mediate the two behaviors. For example, if the square is in
the lateral portion of the fly's field of view at the onset of expansion, the
fly increases stroke amplitude in one wing while decreasing amplitude in the
other, indicative of a collision-avoidance maneuver. In contrast, frontal
expansion elicits an increase in wing-beat frequency and leg extension,
indicative of a landing response. To further characterize the sensitivity of
these responses to expansion rate, we tested a range of expansion velocities
from 100 to 10 000° s-1. Differences in the latency of both the
collision-avoidance reactions and the landing responses with expansion rate
supported the hypothesis that the two behaviors are mediated by separate
pathways. To examine the effects of visual feedback on the magnitude and time
course of the two behaviors, we presented the stimulus under open-loop
conditions, such that the fly's response did not alter the position of the
expanding square. From our results we suggest a model that takes into account
the spatial sensitivities and temporal latencies of the collision-avoidance
and landing responses, and is sufficient to schematically represent how the
fly uses integration of motion information in deciding whether to turn or land
when confronted with an expanding object.

Introduction

When searching for food, a flying animal must efficiently navigate through
its environment, avoid obstacles and eventually alight on its desired target.
Thus, a common choice in any flight search algorithm is the decision about
whether to turn away from an approaching object or land on it. When exploring
their environment, many flies use a series of straight-line flight segments
interspersed with rapid turns called saccades
(Collett and Land, 1975;
Tammero and Dickinson, 2002).
While some saccades are spontaneously generated in the absence of any visual
input (Heide, 1983),
reconstructions of optic flow patterns based upon the fly's motion through an
artificial visual landscape suggest that image expansion plays a role in
triggering saccades (Tammero and
Dickinson, 2002). However, approximations of image expansion have
also been shown to elicit leg extension in tethered flies, which is a motor
response thought to represent a landing reflex
(Borst, 1986;
Goodman, 1960). During
tethered flight in the housefly Musca domestica, leg extension was
accompanied by a change in wing-stroke envelope and a decrease in forward
thrust (Borst and Bahde, 1988).
Thus, as a pattern of visual motion indicative of approaching objects, image
expansion can elicit two potentially conflicting motor responses in the
fly.

Different aspects of the image expansion experienced by a fly might
underlie the decision about whether to saccade or land. First, the visual
processes triggering landing and collision avoidance might have different
sensitivities to the speed of image expansion. Second, the decision to saccade
or land might depend upon differences in the spatial tuning of the two
responses. For example, the focus of image expansion that best activates each
of the two behaviors might lie in different regions of visual field. Third,
information from other sensory modalities, such as the presence of attractive
odors, or the behavioral context, such as the length of the flight period
preceding the decision, might bias the probability with which visual expansion
elicits the two responses.

The purpose of these experiments is to determine which visual cues
available to the fly increase the probability of landing or collision
avoidance. We examine the influence of an expanding object on the fruit fly
Drosophila melanogaster, using a tethered-flight arena in which a
fly's visual environment can be precisely controlled
(Dickinson and Lighton, 1995;
Lehmann and Dickinson, 1997).
When flying within tethered-flight simulators, flies (Drosophila)
exhibit rapid changes in wing kinematics and yaw torque that have been
interpreted as analogous to free-flight saccades
(Götz et al., 1979;
Heide and Götz, 1996;
Heisenberg and Wolf, 1979,
1984). Tethered flies also
demonstrate easily discernable leg extensions that are characteristic of the
landing response (Borst, 1986).
In these experiments, a change in wing stroke sending an object to the rear
visual field is considered a collision-avoidance response. Leg extension is
interpreted as a landing attempt. By examining the effect of both the retinal
position and rate of expansion of the stimulus on the landing and the
collision-avoidance responses, we show that although the stimulus features
that elicit each of these behaviors are similar, each must be processed by
separate circuits within the fly's brain.

Materials and methods

Animals

All experiments were performed on 2- to 5-day-old female fruit flies,
Drosophila melanogaster (Meichen), from a laboratory culture
descended from 200 caught wild females. Flies were tethered with the body in a
hovering posture at a pitch angle of 45° from the vertical, as previously
described (Lehmann and Dickinson,
1997) and were allowed to recover for 1-2h. During this time, the
flies were also dark-adapted, so as to increase their visual responses. Each
set of stimulus presentations lasted between 15-30 min. Any individual that
failed to maintain flight for at least 15 min was not included in further
analysis. The final data set consisted of 41 h of total flight time measured
on 122 flies.

Data collection

The flies were tethered in a virtual-reality flight arena described in
previous studies (Dickinson and Lighton,
1995; Lehmann and Dickinson,
1997) (Fig. 1). The
stroke amplitudes of both wings and the wing-beat frequency were tracked
optically and sampled at 1000 Hz using a data acquisition board (National
Instruments) and software written in MATLAB (Mathworks). The difference
between the left and right wing-beat amplitudes, a signal strongly correlated
to the torque generated by the fly about its yaw axis, was fed back to the
arena controller and used to control the angular velocity of a
15°×15° black square. Thus, the fly actively controlled the
azimuthal position of the square. A sinusoidal bias with a frequency of 0.01
Hz and a maximum amplitude of approximately 75° s-1 was then
added to the feedback signal to make it more difficult for the flies to fixate
the square within the frontal region of the visual field. At 5 s intervals,
the fly was presented with an expansion stimulus in which both dimensions of
the square increased at a constant rate. Thus, we did not systematically map
the response to the expansion stimulus in different regions of the visual
field. Instead, the azimuthal position of the square at the onset of expansion
was determined by the fly, which was actively controlling the position of the
square throughout the experiment. However, because the closed-loop design
produces such long flight sequences, the positions of the expanding stimulus
densely covered the entire range along the azimuth.

Schematized experimental setup for measuring a fly's response to image
expansion. During tethered flight, the fly's wingstroke amplitude and
frequency are measured by optically tracking the shadows cast from an
infra-red (IR) diode by each of the wings on an optical wing-beat analyzer
(Dickinson and Lighton, 1995;
Lehmann and Dickinson, 1997).
During closed-loop experiments, the difference between the amplitude of each
wing stroke controls the visual display, allowing the fly to orient actively
toward the position of the 15°×15° square. At periodic
intervals, the square symmetrically expands, eliciting a behavioral
response.

Because each light-emitting diode (LED) of the arena subtended an angle of
5°, expanding the square symmetrically required a series of 10° jumps
at periodic intervals. The rate of expansion was determined by the constant
interval between 10° jumps. Ten intervals were used, of 100, 70, 40, 30,
20, 10, 7, 5, 2 and 1 ms, which led to expansion velocities of 100, 143, 333,
250, 500, 1000, 1430, 2000, 5000 and 10000° s-1, respectively.
The square expanded until it reached a width of 115° and remained at that
width for 800 ms, after which it instantaneously returned to a width of
15° until the next presentation. 7-15 flies were tested at each rate of
expansion, with each stimulus of a given expansion rate presented to an
individual 150-350 times. The stimulus provided only a simplified version of
the optic flow that a freely flying animal would encounter as it flies toward
a stationary object. For example, in our experiments the expansion rate was
linear and constant, whereas in free flight the rate of expansion would
increase as the animal moves closer to the object (see
Gabbiani et al., 1999). Thus,
our stimulus would simulate a deceleration as the fly approached an object.
Technical limitations due to the low resolution of the visual display and the
method of programming the expansion on it prevented a more naturalistic rate
of expansion.

To measure the fly's landing response, a CCD camera was focused on the fly
and connected to a closed circuit monitor. A photovoltaic sensor was
positioned on the monitor beneath the image of the fly such that extension of
the prothoracic legs would generate an increase in luminance, as sensed by the
diode. The signal from the photovoltaic chip was amplified by a factor of 20
and low-pass-filtered at 10 Hz using a programmable signal conditioner
(CyberAmp 380, Axon Instruments).

Signal conditioning

The raw amplitude and frequency signals from each wing were conditioned for
analysis in the following manner. First, the average wing-beat amplitude
during the 200 ms preceding the presentation of the stimulus was subtracted
from the subsequent response. Each wing-beat amplitude signal was then
smoothed using a low-pass Butterworth filter with a cut-off frequency of 40
Hz. The signals were then downsampled by a factor of 5 to a rate of 200 Hz. To
correct for slight differences in each fly's position over the wing-beat
sensor, the standard deviations of the wing-beat amplitudes from the 200 ms
(an interval representing roughly 40 wing-beat cycles) preceding each stimulus
presentation was calculated over all flies. The standard deviation for each
individual fly was then normalized to this value. To condition the wing-beat
frequency, the mean value of the 200 ms pre-stimulus period was subtracted for
each trace. After that, each value in the trace was divided by the average
pre-stimulus value. Thus, frequency is represented as a percentage of the
baseline level. The wing-beat frequency was then filtered and down-sampled in
a manner identical to the wing-beat signals.

Results

As illustrated in Fig. 2,
expansion of the square to which the flies fixated elicited changes in both
stroke kinematics and leg motion. The expression of these two behavioral
responses varied with the position of the object at the start of expansion.
For example, if the expanding square was positioned in the lateral visual
field at the onset of expansion, the stroke amplitude of the inside wing (that
nearest to the stimulus) increased while that of the outside wing decreased
(Fig. 2A,C). These changes in
wing-beat amplitude caused the object to move caudally in the field of view
(Fig. 2A,C), consistent with a
collision-avoidance response. In contrast, when the square was positioned in
the fly's frontal visual field of view at the onset of expansion, the changes
in wing-beat amplitude were comparatively small, and had little effect on the
position of the stimulus (Fig.
2B). Wing-beat frequency also increased in response to the
expanding square, with the largest increase occurring when the square expanded
directly in front of the fly. Although the exact role of the increase in
wing-beat frequency accompanying the landing response is unknown, it is
thought to represent the fly's attempt to decelerate or generate an upwards
pitching motion. Frontal expansion also elicited leg motion
(Fig. 2B), indicative of a
stereotyped landing response (Borst,
1986; Goodman,
1960).

Wing and leg responses elicited by an expanding object (recorded as V). In
response to a square expanding at a rate of 500° s-1, the fly
generates both wing and leg responses. The time course of stimulus expansion
is shown in the bottom traces. If the object is displaced laterally, the
inside wing (that on the side of the stimulus) shows a transient increase in
wing-beat amplitude, while the outside wing decreases in stroke amplitude. (A)
If the object is to the left of the fly, the left wing-beat amplitude (blue)
increases while the right wing-stroke amplitude (red) decreases, causing the
square to move to the rear of the fly's field of view. In contrast, expansion
of centrally positioned objects elicits smaller changes in wing motion,
causing little change in the position of the object (B). Image expansion in
the frontal field of view elicits leg extension as well as an increase in
wing-beat frequency, both indicative of a landing response. When the stimulus
is to the right of fly, the sign of the change in both wing-beat responses is
reversed, again causing the object to move to the rear of the fly's field of
view (C). Laterally positioned image expansion elicits a transient increase in
wing-beat frequency but does not evoke a leg response.

During free flight, flies avoid collisions using rapid saccadic turns, the
magnitude of which are independent of the fly's angle of approach
(Tammero and Dickinson, 2002).
To determine whether the magnitude of collision-avoidance responses in
tethered flight varied with the retinal position of the expanding object,
individual wing-beat amplitude responses for each fly were grouped by stimulus
position at the onset of expansion (Fig.
3A). Although the size of the responses varied from presentation
to presentation, the average responses elicited by expansion at a lateral
position were larger than those seen at frontal and caudal positions. This
dependency on the position of the expanding stimulus was consistent across
animals (Fig. 3B). The maximum
change in wing-beat amplitude for each wing, the wing-beat frequency, and the
change in the difference between left and right wing-beat amplitudes following
the expansion stimulus, are plotted against stimulus position in
Fig. 4. The change in wing-beat
amplitude for both the left and right wings varies sinusoidally with stimulus
position, as does the maximum change in their difference
(Fig. 4A,B). The largest change
in wing-beat frequency occurs when in the image expands frontally, with a
gradual decay for more lateral stimulus positions. Like wing-beat frequency,
the probability of the expanding object eliciting a landing response is
greatest for frontal expansion (Fig.
4C,D). The two behaviors, changes in wing motion and the landing
response, are not mutually exclusive. Stimulus expansion over a range of
frontolateral positions can elicit both a landing response and a
collision-avoidance response.

The effect of stimulus position on behavioral response. (A) A single fly's
response to multiple presentations of a square expanding at 500°
s-1 varies with stimulus position. Each individual trace shows the
response of the left (blue) and right (red) wing to a presentation of the
expansion stimulus. The bold and dotted lines represent the mean response±
S.D. for stimuli between given positions. Expansion in lateral
positions evokes the largest change in wing-beat amplitude (WBA), with
responses decaying for more frontal and caudal stimulus presentations. (B)
Results from multiple flies. The individual traces are the mean left and right
wing-beat amplitude response taken from 12 individuals. The bold and dotted
lines represent the mean ± S.D., respectively, across individuals.

Collision-avoidance and landing responses vary with the position of
stimulus expansion. (A) The maximum change in value of the wing-beat amplitude
(WBA) from the baseline level of both the right (R; red) and left (L; blue)
wings varies sinusoidally with the position of the stimulus. (B) A similar
variation occurs for the maximum change in the difference between the left and
right wing signals. (C) The percentage change in wing-beat frequency (WBF) was
largest for expansion occurring in front of the fly and decreases slightly for
lateral positions. (D) The probability of eliciting a landing response is
greatest for frontal positions. Data points represent the mean value of
maximum change ± S.E.M. The number of trials at each position is
different because it was determined by where the fly happened to position the
object at the onset of expansion. Data are taken from 300 presentations of a
square expanding at a rate of 500° s-1 to a single fly.

Because the immediate threat of collision with rapidly approaching objects
is greater than that with objects moving more slowly, the fly's response to
image expansion might vary with the rate of expansion. To examine how
different expansion rates affect the collision-avoidance and landing
responses, we measured behavioral changes for squares expanding at varying
rates (Fig. 5). Using the fact
that the difference between the left and right wing-beat amplitudes varies
sinusoidally with stimulus position, we quantified the magnitude of the
collision-avoidance response at each expansion rate by calculating the
amplitude of a sine wave fitted to the position-response curve for each fly.
To quantify the landing response at each expansion rate, we determined the
width of the range of stimulus positions in which the probability of landing
response was greater than 0.5 for each fly. We normalized the response by
dividing the sine amplitudes and the landing response widths measured for each
rate of expansion by the maximum mean responses
(Fig. 6). Although the
sensitivity of the two behaviors to expansion rate is similar, the
collision-avoidance response is more broadly tuned. Whereas the two behaviors
are maximally activated at an expansion rate of approximately 1000°
s-1, the collision-avoidance response displays a greater
sensitivity to both faster and slower expansion than the landing response.

The effects of expansion rate on collision-avoidance and landing responses.
Each column represents the wing-beat amplitude (WBA), and the landing response
probabilities plotted against stimulus position as described in
Fig. 4 for a different rate of
expansion. The functions shown in Fig.
4 were determined for each fly, with each data point representing
the mean ± S.E.M. taken over all the flies. The numbers of flies tested
were 8, 11, 11, 8, 12, 11, 8, 10, 7, 7 and 5 for expansion rate in ascending
order (starting at 100° s-1). The total numbers of stimulus
presentations, again in ascending order, were 1945, 2905, 2584, 1746, 3237,
2529, 216, 2223, 1771, 1391 and 1132. The sinusoidal shape of the wing-stroke
amplitude responses holds for all expansion rates, with the amplitude of the
response being largest for an expansion rate of 1000° s-1. The
probability of landing is high over the greatest range of positions at an
expansion rate of 1430° s-1. L, left; R, right; WBF, wing-beat
frequency.

Summary of changes in collision-avoidance and landing responses with rate
of image expansion. The collision-avoidance response for a given expansion
rate (open circles) is the sinusoid amplitude best fitting the maximum change
in the difference between wing-beat amplitudes (see
Fig. 5, second row), normalized
by the maximum mean amplitude. The width of the range of positions for which
the probability of landing is greater than 0.5 characterizes the landing
response for a given expansion rate (filled circles). This response is
normalized by the maximum mean width value. Values are means ± S.E.M.
for each fly.

A previous study showed that collision-avoidance saccades occurring during
free flight are of constant magnitude, suggesting a pre-programmed motor
response (Tammero and Dickinson,
2002). In contrast, the amplitude of collision-avoidance responses
during tethered flight varied with both stimulus position and expansion
velocity. To examine the effect of expansion speed on the time course of the
collision-avoidance response, we plotted the difference between the left and
right wing-beat amplitudes for stimuli of varying rates of expansion occurring
at three locations (Fig. 7).
For stimuli in each location, both the duration and amplitude of the response
rose with the rate of expansion to a maximum at 1000° s-1 and
fell off for faster rates. The influence of stimulus position on the response
was small when compared to the effects of expansion rate.

Effect of stimulus position and expansion rate on the time course of the
wing response. Responses to stimuli presented within ± 10° of the
position were pooled. Each trace represents the mean ± S.D. (shaded
area) of the average responses taken from multiple flies. The time course of
the responses does not vary with stimulus position but does vary greatly with
rate of expansion. The number of flies at each expansion rate is given in
Fig. 5. L, left; R, right.

In addition to influencing response amplitude, variation in stimulus
position and rate of expansion might also affect the delay between the start
of expansion and the onset of the response. In
Fig. 8A,B, the latency from the
start of expansion to the onset of both the collision-avoidance and landing
responses is plotted. The latency of the collision-avoidance response shows a
relatively constant value of approximately 50 ms for stimulus positions
between ±50° and ±130°, but increases for more frontal
and caudal positions. In contrast, the landing response shows a nearly
constant delay of approximately 150 ms at all retinal positions where
expansion can trigger a landing response. Thus, whereas the probability of
generating a landing response does depend on stimulus position, the latency of
the landing response does not. To examine the effect of expansion rate on
response delay, the minimum latencies for the collision-avoidance and landing
responses were plotted against rate of stimulus expansion
(Fig. 8C). Minimum landing
response latency falls asymptotically from a value of 300 ms at 100°
s-1 to a value of 100 ms at rates of 1000° s-1 and
higher. In contrast, the minimum collision-avoidance delay shows a relatively
constant value of approximately 50 ms at all but the lowest expansion
rates.

Collision-avoidance and landing response latencies depend on stimulus
position and expansion rate. Latency is measured as the time interval between
the onset of image expansion and the initiation of the landing or
collision-avoidance response. (A) Latency in response to expansion at a rate
of 500° s-1 is relatively constant over lateral portions of the
fly's field of view and increases for positions to the front and rear. Data
points represent mean latency ± S.E.M. for 12 flies. (B) Landing
response latency to a square expanding at 500° s-1 is constant
at the stimulus positions at which landing response probability is high. At
this expansion rate, the collision-avoidance latency is approximately half
that of the landing response. (C) Response latencies plotted as a function of
expansion rate. For a given rate of expansion, the minimum of the mean delay
functions (such as the two plotted above) was determined. Filled circles
represent the minimum mean delay in the landing response, while empty circles
represent the minimum mean delay of the collision-avoidance response. The
landing response latency decreases with the rate of expansion, whereas for
most expansion rates the delay of the collision-avoidance response is
constant.

In the experiments described, each fly controlled the position of the
square both before and during stimulus expansion. Therefore, the animal's
response to the stimulus altered the expansion to which it was subject. To
determine if this closed-loop implementation affected the behavioral
responses, we presented expansion stimuli under open-loop conditions in which
the fly's behavior had no impact on the position of the stimulus. A comparison
of the open- and closed-loop responses for stimuli with identical expansion
rates is depicted in Fig. 9.
Instead of varying sinusoidally with stimulus position, the relationship
between stimulus position and the difference in the left and right wing beat
signals is better approximated by a square wave or a sigmoid in the open-loop
case (Fig. 9A). In addition,
the probability of the landing response is slightly reduced compared with the
closed-loop case (Fig. 9B). The
latencies of the landing and collision-avoidance responses are similar under
closed- and open-loop conditions, although the landing response delay is
slightly longer following an open-loop presentation
(Fig. 9C,D). The individual
wing responses follow similar time courses
(Fig. 10), although the
responses in both wings are larger in the open-loop case.

Open-loop versus closed-loop responses to image expansion. (A)
During open-loop presentation the position of the square was controlled
externally, as opposed to the closed-loop paradigm in which the fly maintains
control over the position of the square. The closed-loop wing responses
(filled circles) are repeated from Fig.
4. The open-loop responses (open circles), also generated in
response to an expansion at a rate of 500° s-1, vary roughly
with stimulus position as a square wave, in contrast to the open-loop
responses, which vary sinusoidally. Thus, the ability to control the position
of the square during the collision-avoidance reaction does affect the
amplitude of the response. (B) The probability of landing is slightly reduced
for open-loop presentations. (C) The latency of the collision-avoidance
response is qualitatively similar for the closed- and open-loop stimuli, with
slightly larger latencies in response to open-loop image expansion. (D) The
open-loop landing response latencies were qualitatively similar to those seen
during closed-loop presentations. Again, the latency is slightly shorter
during closed-loop presentations. WBA, wing-beat amplitude.

Comparison of the time course of responses for closed-loop and open-loop
presentations. The changes in wing-beat amplitude (WBA) in response to a
square expanding at 500° s-1 positioned between -140° and
-120° followed a similar time course for open- and closed-loop
presentations. The responses elicited by closed-loop presentation of the
square were slightly smaller in magnitude than those in response to open-loop
presentations. Closed-loop responses were taken from
Fig. 3B; open-loop responses
were taken from 5 flies in a manner analogous to the data plots in
Fig. 3B.

The open- and closed-loop responses to a uniformly expanding object
demonstrate that image expansion is a sufficient stimulus for eliciting
collision-avoidance and landing responses in tethered flight. The amplitudes
and latencies of these responses depend in part on both the position of the
stimulus at the onset of expansion and the rate of expansion.

Discussion

The results indicate that an expanding object elicits both
collision-avoidance and landing responses in tethered Drosophila.
Although the same stimulus may elicit both behaviors, several observations
suggest that different pathways mediate the two reactions. First, the
azimuthal position of the stimulus affects the expression of the two behaviors
in different ways. For example, a fly generates its strongest
collision-avoidance reaction as a result of image expansion in the lateral
portions of its visual field (Figs
2,3,4,5),
whereas the probability of landing is greatest for stimuli in a frontal
position (Figs 2,
4,
5). Second, although both
responses are most sensitive to rates of image expansion of approximately
1000° s-1, the collision-avoidance response is more broadly
tuned, showing strong reactions for a greater range of image velocities than
the landing response (Figs 5,
6). Third, whereas the time
course of the collision-avoidance response varies with rate of expansion
(Fig. 7), the time course of
the landing response remains constant. Fourth, the latency of the
collision-avoidance reaction maintains a relatively constant value of 50 ms
for different expansion velocities (Fig.
8). In contrast, the delay before the onset of the landing
response was larger than that of the collision-avoidance response (between
100-300 ms) and shows a larger variation with rate of image expansion
(Fig. 8). Finally, whereas the
landing response appears to represent a true fixed action pattern, the
collision-avoidance response is influenced by feedback. Removing the fly's
control over the position of the stimulus did not alter the time course of the
collision-avoidance reaction or the latency of either response, but did
increase the amplitude of the avoidance response for stimuli in caudolateral
positions (Figs 9,
10).

Are collision-avoidance reactions and torque spikes the
tethered-flight analogs of free-flight saccades?

Many studies have described the rapid changes in wingstroke amplitude
(sometimes referred to as `wing hitches') and the torque spikes generated
during tethered flight in Drosophila
(Götz et al., 1979;
Heide and Götz, 1996;
Heisenberg and Wolf, 1979;
Heisenberg and Wolf, 1984).
Superficially, the tethered-flight collision-avoidance reactions seem similar
to free-flight saccades. Both are visually elicited responses and both direct
the fly away from approaching objects. Closer inspection of the two behaviors
reveals important differences, however. Reconstructions of 3-dimensional
free-flight trajectories taken at 30 frames s-1 suggest that the
saccades generated by Drosophila are ballistic turns, lasting no
greater than 100 ms, during which a fly's heading is altered by 90°
(Tammero and Dickinson, 2002).
In contrast, the changes in wing stroke evoked by an expanding square in
tethered flight last 600-700 ms (Fig.
3), roughly 12-14 times the length of a free-flight saccade. In
addition, high-speed video recordings of free flying animals at 5000 frames
s-1 indicate that there is little change in the wing-beat amplitude
during the course of the saccades (S. Fry and M. H. Dickinson, unpublished
observations). This subtle alteration in wing kinematics is in contrast to the
large and long-lasting change in left and right stroke amplitude during
collision-avoidance responses seen in tethered flight. The discrepancies in
the wing and body kinematics of tethered-flight collision-avoidance reactions
and free-flight saccades question the assumption that the two are analogous.
One possible explanation for the differences between the tethered-flight
reactions and free-flight saccades is that tethering a fly interrupts
mechanosensory feedback from the halteres (gyroscopic sensors sensitive to
angular velocities about the fly's roll, pitch and yaw axes)
(Dickinson, 1999;
Nalbach, 1993;
Nalbach and Hengstenberg,
1994). Mechanosensory feedback from the halteres, antennae, the
wings and other sensors during the initial stages of a saccade might serve a
critical role in turning off the motor program that alters wing kinematics,
thus reducing the duration and magnitude of the saccade. Restoring some
mechanosensory information by allowing a `loosely tethered' fly to rotate
freely about its yaw axis reduces the duration of a tethered saccade from 700
to 250 ms (Mayer et al.,
1988).

Visual feedback during collision-avoidance reactions

Previous studies have examined the role that visual feedback plays during
the course of a saccade. Spontaneously reversing the direction of displacement
of a visual object increased the length of torque spikes, whereas doubling or
eliminating the displacement in the expected direction had little effect
(Heisenberg and Wolf, 1979,
1984). These results suggest
that visual information alters the saccade only if the direction of motion is
opposite to what is expected during the course of the torque spike. In our
experiments, the systematic variation in the amplitude of the
collision-avoidance response with the position of the square and the
difference between open- and closed-loop responses suggest that visual
feedback does play a role in regulating the size of the motor response. There
are two possible explanations for variations in the size of the
collision-avoidance reaction. First, the fly's nervous system sends different
commands to the motor system in response to image expansion occurring at
different positions in the visual field, with the fly following these commands
in a feed-forward maneuver. Alternatively, the nervous system might issue a
single avoidance command and the variation in the response amplitude reflects
the role of sensory feedback. Removing the fly's control over the position of
the square by presenting the stimulus in open loop resulted in larger
responses (Fig. 10),
particularly for positions at the rear of the fly's visual field. It is
unlikely that the identical visual stimuli presented in similar locations
during our experiments would elicit different commands from the fly's nervous
system. Although other sensory modalities, such as olfaction, might be able to
modify the command sent to the motor system, during our experiments each
presentation was made under identical circumstances, thus minimizing any
effects that other sensory modalities could have on the motor command. Because
the fly's collision-avoidance reaction causes the square to move to the rear
of the fly's field of view, the expansion the fly experiences is reduced,
leading to a smaller response. These results are consistent with prior
observations showing that free-flight saccades are slightly larger when the
flies fly within a uniform visual panorama, compared to those generated in a
rich-textured background (Tammero and
Dickinson, 2002). Thus, our results are best explained by a model
in which the motor response following a saccade command is modulated by
feedback from the visual and mechanosensory systems.

Responses to image expansion in other insects

Although collision-avoidance responses have not been previously reported in
tethered flies, neurons sensitive to image expansion have been described in
flies and other insects. Neurons sensitive to frontally positioned
approximations of image expansion have been described in the cervical
connective of the blowfly Calliphora erythrocephala, and are thought
to play a role in generating the landing response
(Borst, 1991). In the locust
Schistocerca americana, descending contralateral motion detector
cells (DCMDs) may play a roll in collision-avoidance or escape behavior by
firing in response to looming objects
(Gabbiani et al., 1999;
Gray et al., 2001;
Judge and Rind, 1997). In the
lobula plate of the hawk moth Manduca sexta, two cells have been
identified that respond to looming stimuli. Class 1 cells respond to changes
in the size of the looming object, and class 2 cells respond to an expanding
optic flow field (Wicklein and Strausfeld,
2000). At present, there is no way of knowing whether the
homologues of any of these cells are responsible for the collision-avoidance
behavior, although the properties inferred from the behavior do suggest that
the Drosophila cells represent a new class.

Mechanisms underlying the collision-avoidance and landing
responses

To detect the expanding square and trigger the collision-avoidance and
landing responses, flies might perform several different neural calculations.
One possibility is a `time-to-contact' model, where the fly calculates time
before a collision with the square, and either saccades or extends its legs
before the anticipated contact. A second possibility is a `temporal contrast'
model, in which the fly responds to darkening in its field of view.
Alternatively, a fly may generate a response when the image across its retina
subtends a certain width or area, which we will refer to as a stimulus size
trigger. Finally, the fly might integrate image motion over space and time,
with saccade initiation occurring when the integral exceeds a threshold,
referred to as the `spatio-temporal integration model'
(Borst, 1990;
Borst and Bahde, 1988).

The time-to-contact model has been proposed as the trigger for deceleration
before landing in freely flying Drosophila
(Wagner, 1982). However, such
a calculation cannot be responsible for triggering the collision-avoidance
response in our experiments, as the latency of the response is uniform for
varying rates of expansion, which approximate different approach speeds and
thus different times-to-contact (Fig.
8). Although landing-response latency does vary with expansion
rate (Fig. 8), in other species
of flies this latency also depends upon image contrast and size, factors that
are inconsistent with the time-to-contact model
(Borst and Bahde, 1986;
Eckert and Hamdorf, 1980).
Decreases in temporal contrast (i.e. darkening), coupled with object motion,
evoke escape responses in stationary Drosophila
(Holmqvist and Srinivasan,
1991; Trimarchi and
Schneiderman, 1995). In our experiments, however, large changes in
temporal contrast, generated by instantaneous increases in the size of the
square, elicited neither collision-avoidance nor landing responses
(Fig. 5). Cells that respond
when an expanding object reaches a certain size have been described in locusts
(Gabbiani et al., 1999,
2001) and hawk moths
(Wicklein and Strausfeld,
2000). Such a model has also been suggested for
Drosophila (Wittekind,
1988), yet other investigators have demonstrated that larger flies
will land in response to sinusoidal gratings whose total size remains constant
(Borst and Bahde, 1986;
Wehrhahn et al., 1981). In our
experiments, response latency did not vary with expansion rate, as would be
expected if the response were triggered by an absolute stimulus size. Thus,
the stimulus-size model cannot account for our results. A model in which the
spatially and temporally integrated output of local motion detectors exceed a
threshold to trigger a response has previously been proposed to account for
landing behavior in flies (Borst,
1990), and remains the most parsimonious explanation for the
behavioral responses described here.

Optic flow model for saccade and landing response initiation

Many studies have emphasized the important role that optic flow plays in
the control of insect flight (Collett et
al., 1993; Krapp et al.,
1998; Srinivasan,
1993). Flies are thought to estimate optic flow by means of a
retinotopic array of motion detectors, each of which provides information on
the amplitude and direction of motion occurring over a small portion of a
fly's visual field. By spatially integrating their inputs according to
appropriate `matched filters', a fly receives feedback about its translational
and rotational movement by spatially integrating responses from individual
local motion detectors (Franz and Krapp,
2000; Krapp et al.,
1998). A model in which estimation of optic flow information is
used to initiate collision-avoidance and landing responses is shown in
Fig. 11. Output from local
motion detectors is appropriately pooled, to measure image expansion occurring
over different regions of the fly's visual world. Independent initiation of
the two behaviors requires that image expansion be calculated over at least
three different regions, the lateral left, lateral right, and frontal fields
of view. Because the collision-avoidance reactions and landing responses are
discrete events, both are likely to be triggered when some neural signal
exceeds a threshold. Temporal integration of the expansion signal would allow
the input signal to accumulate even when the response of a hypothetical
expansion cell has reached a steady-state level. This allows the signal to
exceed this threshold, while at the same time beneficially conditioning the
signal, making the input to the threshold detector less sensitive to high
frequency noise. This temporal integration must be `leaky', as the weak motion
stimuli do not elicit responses (Borst,
1990).

Model for eliciting collision-avoidance and landing responses. A fly
estimates the optic flow experienced during flight using a two-dimensional
array of motion detectors (i). Local motion information is then spatially
pooled such that the image expansion in both the lateral and frontal fields of
view is calculated (ii). The outputs of each of these three expansion
calculations are then temporally integrated (iii) and passed through a
threshold detector (iv). Expansion detected in a lateral field of view
triggers a collision-avoidance response in the opposite direction, while
frontal image expansion causes a landing response (v). Lateral expansion on
one side inhibits the opposite expansion pathway, preventing a saccade from
being immediately followed by another saccade in the opposite direction.

The longer latencies associated with the landing response when compared to
collision avoidance can be explained either by differences in visual
processing or in the speed with which the motor system responds upon receiving
a descending command from the brain. It is unlikely that the longer latency of
the landing responses is due to slower activation of the motor system, as
studies on the flight initiation in Drosophila demonstrate that the
tibial levator muscle is activated as rapidly as 1-2 ms after activation of
the giant fiber (Trimarchi and
Schneiderman, 1993). During visually elicited flight initiation,
leg extension occurs approximately 20 ms after the presentation of the
stimulus (Trimarchi and Schneiderman,
1995). Additionally, the expansion-sensitive neurons in the
cervical connective of the blowfly Calliphora erythrocephala respond
to bilateral image expansion with a latency between 100 and 200 ms
(Borst, 1991), a value close to
the latency of the landing response in Drosophila and Musca
(Borst, 1986). Thus, it is
likely that the longer latency in the initiation of the landing response
reflects a difference in the time required for the spatial and temporal
integration of the visual signal, suggesting that separate circuits mediate
detection of the visual stimuli that trigger the collision-avoidance and
landing responses. The longer latency of the landing response as compared to
the collision-avoidance reaction may indicate either that a higher threshold
level must be surpassed to trigger a response, or an increased amount of
leakiness in the integrator preceding the threshold detectors in the landing
system. Leakiness in the integrator can also explain the larger sensitivity of
the landing response latencies to expansion rate. Because it would be
behaviorally disastrous if a saccade in one direction was followed immediately
by a saccade in the opposite direction, the output of the lateral expansion
detectors inhibits the opposite expansion pathway in our proposed model. Where
in the collision-avoidance pathway this inhibition is manifested could not be
determined by these experiments.

In our experiments, we varied only the azimuthal position of the expanding
square. The center of the square was fixed along the equator of the fly's
visual field. If the elevation of the square changed from this position, it is
unlikely that the output of the model would be changed, particularly when the
horizontal edges of the expanding object were on opposite sides of the equator
of the fly's field of view. Our experiments did not examine the relative
importance of the vertical and horizontal components of the expansion.
However, in a study of the landing response in the housefly Musca
domestica, the directional sensitivity of the landing response was
dependent on the position of the stimulus in the fly's visual field
(Wehrhahn et al., 1981). In
the frontal visual field above the equator, motion in the upward direction
initiated the landing response most strongly, whereas in the lateral visual
field at the equator, landing was most strongly initiated by front-to-back
horizontal motion. Our model, based upon the spatio-temporal integration of
optic flow, would predict similar results.

Furthermore, our model does not consider changes in response that might
occur with multiple presentations of a stimulus. The landing response in
Drosophila does attenuate with multiple presentations
(Fischbach, 1981). We saw no
attenuation in the collision-avoidance response during the course of our
experiments. One possibility is that the azimuthal position of the square
varied for each trial, preventing multiple repeated presentations in the same
location. Even during the open-loop presentations, the azimuthal position of
the square varied randomly. However, during our experiments the square
expanded at approximately 5 s intervals, whereas in free flight visually
induced collision-avoidance saccades occur at intervals of 0.75-1.5 s
(Tammero and Dickinson, 2002)
over flight trajectories lasting several minutes. Although decreasing the time
between expansions while holding the position of the square constant might
reveal some habituation, our experiments as presented did not result in
noticeable habituation, and thus this feature is not included in the
model.

Optic-flow model and free-flight behavior

The visual information that the fly receives from the expanding square in
our experiments differs from what it would receive if it were freely flying
towards an object at a constant velocity. During our experiments, the square
expanded at a constant rate, which during free flight would result from the
fly decelerating as it approached the object. Trajectories from free-flight
experiments have demonstrated that flies do decelerate as they approach the
walls of the arena (L. Tammero and M. Dickinson, unpublished data). However,
the stimulus used in our tethered experiments is only an approximation of the
image expansion experienced during free-flight object approach. Despite this,
the stimuli still reliably initiated both collision-avoidance and landing
responses. The theoretical model discussed previously would respond in a
similar fashion to objects expanding at a constant rate as well as to objects
whose rate of expansion increased exponentially.

During free flight, a fruit fly explores its environment using a series of
straight line segments interspersed with saccades
(Tammero and Dickinson, 2002).
Given that similar stimuli can evoke either a collision-avoidance response or
a landing response, how is it that these behaviors do not interfere with one
another in free flight? Reconstructing the fly's estimation of optic flow
during free flight suggested that image expansion in the frontolateral field
of view precedes each saccade. Although in tethered flight, expansion in
frontolateral portions of the fly's field of view could elicit either a
saccade or a landing response, the latency before the onset of the saccade is
shorter than the landing response latency. Thus, if a freely flying
Drosophila were to experience image expansion capable of eliciting
both a saccade and a landing response, the saccade is likely to occur first,
sending the expanding image to the rear of the fly's field of view. During the
period of straight flight following the saccade, our model predicts that the
expansion experienced by the fly builds until the next saccade is triggered. A
landing that terminates the flight trajectory could be elicited by image
expansion occurring in the central portion of the fly's field of view, or by
inhibition of the fly's saccade pathways.

In free flight, the focus of expansion at which image velocity is zero
should reside in the fly's frontal visual field, assuming the animal
translates while keeping its body axis tangent to its flight path and does not
rotate. During the tethered-flight experiments, however, the
collision-avoidance responses were initiated by a square expanding in more
lateral positions of the fly's visual field. For the image expansion reflexes
identified in these experiments to initiate saccades during free flight, the
focus of expansion must be displaced to a more lateral position in the visual
field. This lateral displacement may result from either rotation about the
fly's yaw axis or side-slip. Alternatively, the collision-avoidance reflexes
elicited by lateral expansion might represent circuitry that functions to
detect moving objects such as predators, whereas free-flight saccades elicited
by the fly's approach to a static background might be initiated by separate
reflexes not yet identified. High-resolution free-flight tracking, in which
the body orientation as well as the position of the fly is visualized, will be
necessary to differentiate between these two alternatives.

Image expansion plays a central role in the initiation of landing responses
and collision-avoidance reactions and, by extension, saccades. Variations in
both responses with the retinal location of image expansion, and differences
in the latencies associated with visual processing, explain how the landing
and collision-avoidance reactions interact during free flight. Because
saccades seem to be the prevalent means by which flies alter flight heading,
the initiation of these saccades plays a large role in controlling their
flight behavior. Variations in the constituent elements of our conceptual
model (such as the threshold levels or leakiness of the temporal integrator)
would necessarily be reflected by the animal's behavioral output. Thus, the
computations using image expansion to land on and avoid approaching obstacles
is a clear example of how patterns of behavior might emerge from interactions
between the animal and its environment.

ACKNOWLEDGEMENTS

The authors wish to thank Jocelyn Staunton and Jessica Choe for maintaining
the fly colonies. This work was supported by grants from the National Science
Foundation (FD97-23424), ONR (FDN00014-99-1-0892) and Darpa
(N00014-98-1-0855).

Heide, G. and Götz, K. G. (1996).
Optomotor control of course and altitude in Drosophila melanogaster
is correlated with distinct activities of at least three pairs of flight
steering muscles. J. Exp. Biol.199,1711
-1726.

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